Autocross nerd thread: The Ultimate E9X A-Stock SCCA Autocross Build?

[Richbot]

With the new Street rules the car to have is a ZCP stripper coupe. ZCP so you can run the wider wheel width downsized to 18", and get the lower ride height. If not a stripper, no sunroof and no nav/super stereo are the most weight critical options it seems. Trans is probably a wash but in terms of absolute speed 6MT is going to be 40lb lighter so that's what i would do.

[m3peat]

Originally Posted by Richbot

With the new Street rules the car to have is a ZCP stripper coupe. ZCP so you can run the wider wheel width downsized to 18", and get the lower ride height. If not a stripper, no sunroof and no nav/super stereo are the most weight critical options it seems. Trans is probably a wash but in terms of absolute speed 6MT is going to be 40lb lighter so that's what i would do.

Any wheels you have mind for the 18's? I have a zcp stripped a bit but not a stripper.
I would love to get some 18's that look good and are light w/o breaking the bank but still streetable.

[Black Gold]

Apex ec-7

[m3peat]

Originally Posted by KennyPowers

Apex ec-7

Those would work and look good! Thanks

wish they trimmed more weight but 4-5 lbs each isn't bad.

[CSBM5]

Originally Posted by m3peat

Those would work and look good! Thanks

wish they trimmed more weight but 4-5 lbs each isn't bad.

You can run their front 8.5"/ET35 with a 12mm spacer yielding 23mm net offset and be just inside the SCCA maximum allowed (1/4", 6.35mm) offset change from stock. Their rear 9.5"/ET22 would work perfectly as is. Stock offsets are 29mm/23mm.

[Richbot]

Originally Posted by CSBM5

You can run their front 8.5"/ET35 with a 12mm spacer yielding 23mm net offset and be just inside the SCCA maximum allowed (1/4", 6.35mm) offset change from stock. Their rear 9.5"/ET22 would work perfectly as is. Stock offsets are 29mm/23mm.

For ZCP he needs 9s and 10s. Street rules allow downsize of diameter but you get to keep the width. The 18x10 et25 ec7 should work for the rear and I think they have a 9" front 1swries fitment which would work with a spacer

[IDreamOfEvos]

Great thread

[CSBM5]

Originally Posted by Richbot

For ZCP he needs 9s and 10s. Street rules allow downsize of diameter but you get to keep the width. The 18x10 et25 ec7 should work for the rear and I think they have a 9" front 1swries fitment which would work with a spacer

Ah, I totally missed that part. Yeah, they do make a 9x18 ET31 that could be used up front along with a 10x18 ET25 for rears.

ZCP offsets are 31/25, so no spacer equals stock offsets all around.

[jcp907]

In looking at cars, I am finding some pre '11 with a ZCP retrofit. However, in reading the latest 2014 draft, I don't believe that would be legal to install on an older car. For instance, if we started with an 08-10 and "retrofitted" with the ZCP package, that would not be legal for F Street. If this is the case, that makes the 11+ more desirable. Am I understanding this correctly?

Also, I would believe that since the E90 CRT and the Lime Rock editions were not produced in a quantity of 1,000 or more, neither would be legal in the new street class. This seems pretty straightforward, but I want to be certain that I am not overlooking other models that might be legal.

[CSBM5]

Originally Posted by jcp907

In looking at cars, I am finding some pre '11 with a ZCP retrofit. However, in reading the latest 2014 draft, I don't believe that would be legal to install on an older car. For instance, if we started with an 08-10 and "retrofitted" with the ZCP package, that would not be legal for F Street. If this is the case, that makes the 11+ more desirable. Am I understanding this correctly?

Yes, I think so.

Re ZCP, if you're very serious about a top-line competitive car, you'll be replacing shocks anyway, so the benefits of ZCP will be 1/2" wheel width increase front and rear and the different springs. I would guess the wheel width increase is likely the best of the two, but now that you can change diameter, it makes it an even better idea. However, I wouldn't think that a non-ZCP car would be non-competitive at all (i.e. the minor spring change and 1/2" wheel width might well be within the bounds of repeatability in testing on typical autox courses).

In my M3 hunt, once I decided on the newest car I could find, I discovered that it would be nice to find a 2011.5 or newer car for nothing to do with autocross (i.e. nice interface with iphone, bt music streaming, and a few other things). It was only an accident that I was able to find one as my main search was for a 6MT and no sunroof (only about 200 2011 E90 M3s were built this way).

As far as the E90 goes, I think I have the lightest setup (6MT, no sunroof -- need this for helmet clearance big time, no fold-down seats, no EDC, but most all other options, which weigh little...i.e. I wanted iDrive, heated seats, comfort access, etc).

The 6MT saves 50lbs over DCT. The non-sunroof saves something in the range of 35-40lbs (hard to find in E90s). Not sure how much non-EDC saves, a bit I guess, and I love the damping rates of the stock shocks btw.

[Richbot]

iDrive adds about 10lb due to computer, camel hump and screen and some associate wiring. Plus that controller weighs like 150lbs. My E90 is lighter Nyah-nyah

Agree stock shocks are very close and upgrade is not a necessity for a good build. But for adapting to changing conditions I found them to be pretty limiting as you're stuck with tire pressures and front bar which are not as fertile ground for improvement of balance like a damping change can be.

LRP:. The m performance exhaust is the only thing affecting weight or performance relative to a plain jane ZCP and cat backs are unrestricted in stock. Badge and tape jobs like the LRP edition have a history of making it through the protest process.

Retrofit of ZCP is not legal. Half inch of wheel width and lower ride height are probably worth it. Before the wheel rule I would have called it a wash because you can fit wider fronts on the 18's available for non-ZCP cars, but now that you can downsize the ZCP is definitely the car to have. While a retrofit is not legal, converting an '11.5+ to ZCP would be legal so long as you do everything, which is really just springs dampers and wheels, two out of those three you are going to be changing anyway for a full-tilt build. You'd have to update the stability control and EDC programming as well though the EDC programming would be a gray area as you can disable adaptive dampers under the rules by fitting afyermarket non adaptive units.

The CRT and GTS would not be eligible for stock because they were never sold here in North America. The 1000 examples requirement is for cars which are in the not otherwise classified category. The 1M for instance is legal for stock/street in spite of low numbers because it was specifically classed on its own line

[CSBM5]

jcp907,

Here ya go, right here on the board:
http://www.m3post.com/forums/showthread.php?t=928381

[jcp907]

Originally Posted by CSBM5

However, I wouldn't think that a non-ZCP car would be non-competitive at all (i.e. the minor spring change and 1/2" wheel width might well be within the bounds of repeatability in testing on typical autox courses).

And either would be better than an M5-at autocross!

Originally Posted by CSBM5

The 6MT saves 50lbs over DCT..and I love the damping rates of the stock shocks btw

I would certainly run the stock shocks for a year or so, at minimum. Money would go to wheels and driving schools first. Please let me know when you will be teaching up your way, Chuck. I'd like to take advantage of learning from you! Also, I am glad to hear that the shocks are well liked, by you.

The consensus is the M6 trans so far, but, to play devil's advocate: Looking at this from a statistical point of view, the DCT offers 5% torque advantage in 2nd gear (from gearing-see below). A number that probably shouldn't be ignored, considering this could be 115 ft lbs to the tire. Granted parasitic losses aren't calculated-are they similar between the two transmissions?

Can the M3 put this torque down sufficiently with 295 street tires? Or is there a traction issue as it sits?

Also, shift times are much faster with the DCT-can this be quantified?

Gearing analysis:
M6 1st gear final drive: 4.055*3.678=14.91429 ORIGINAL POST INCORRECT REAR GEAR
M6 2nd gear final drive: 2.396*3.678=8.812488 ORIGINAL POST INCORRECT REAR GEAR
DCT 1st gear final drive:4.78*3.154=15.07612
DCT 2nd gear final drive:2.933*3.154=9.250682

M6 1st Gear final drive: 4.055*3.846=15.59553 CORRECT REAR GEAR
M6 2nd Gear final drive: 2.396*3.846=9.215016 CORRECT REAR GEAR

ORIGINALLY POSTED WITH INCORRECT GEAR INFORMATION
Also, this doesn't take into consideration any torque converter torque multiplication (or slip).
That is a 1% advantage in 1st gear...meh
But, it's a 5% torque advantage in 2nd gear, with a 50 lb penalty. This should be measurable an autocross. Would you disagree?

With the correct gear information in mind, there is no significant gearing advantage with the DCT. The conclusion would be to go with the manual, which is 50 lbs lighter.

Although, one would still be able to left foot brake more effectively (arguably) with only 2 pedals, particularly with rev matching-assuming the software works properly.


OLD CONCLUSION BASED ON INCORRECT GEAR DATA:
After looking at this, the DCT looks to be measurably better to me (for autocross reasons only), even with the 50 lb penalty. Additionally, this weight is in the middle of the car, very close to CG.

Please shoot holes in this, because I'd prefer the M6, but, if the DCT is measurably better, that's the way I'd need to go...if I were to do this. Of course, if the competition package is only available with the M6...then the comp package is the way to go. Also, if there were still DCT issues, that would steer me away from that. I don't think that is the case, however. Thoughts?

Thank you Rich and Chuck for setting me straight!

[jcp907]

Originally Posted by CSBM5

jcp907,

Here ya go, right here on the board:
http://www.m3post.com/forums/showthread.php?t=928381

Chuck, you've heard of analysis paralysis, right? I am not done figuring! lol

No, really, I probably can't pull the trigger until June. Child support on my 17 year old ends. That's the car PLUS Insurance payment.

Regardless, I just ordered another set of rear wheels and tires will be coming soon for the M5 that will get me through some driving schools. I am considering a driving instructor for the season-in the M5 as well.

[Richbot]

Originally Posted by jcp907

Chuck, you've heard of analysis paralysis, right? I am not done figuring! lol

No, really, I probably can't pull the trigger until June. Child support on my 17 year old ends. That's the car PLUS Insurance payment.

Regardless, I just ordered another set of rear wheels and tires will be coming soon for the M5 that will get me through some driving schools. I am considering a driving instructor for the season-in the M5 as well.

6mt has. 3.85 rear end gear not 3.68. Your figures are likely taken from an incorrect source which seems to end up high on the m3 gear ratios google result. It's 3.85:1 for the 6Mt i have verified rotations myself and this has also been verified many times over through speed in gear calcs ad nauseum
ISuffice to say, the car would do 179 mph in 5th if it had a 3.68 rearend and it doesnt...
See also bmwusa specs for the m3 vert. Same parts.

Torque multiplication in 2nd is within 0.5 percent and that does not account for the dct tending to dyno lower by about a dozen hp

[jcp907]

Originally Posted by Richbot

6mt has. 3.85 rear end gear not 3.68. Your figures are likely taken from an incorrect source which seems to end up high on the m3 gear ratios google result. It's 3.85:1 for the 6Mt i have verified rotations myself and this has also been verified many times over through speed in gear calcs ad nauseum
Torque multiplication in 2nd is within 0.5 percent and that does not account for the dct tending to dyno lower by about a dozen hp

Excellent! Thank you very much for the clarification. I am sorry that I quoted incorrect information, and, thank you for setting me straight!

[CSBM5]

You'd be surprised how much closer the two cars are on an open track like the Performance Center events I've taken them both to. The M3 on the same tires is clearly faster, but it's not like a beat down. Now on more normal autocross events with a lot of slower speed stuff, slaloms, etc, the M3 just destroys the M5.

One of the biggest differences you'll find is how well the M3 can put down power. The S65 torque curve is so flat over a very broad range of rpms, and therefore the hp builds so progressively into that vicious crescendo from 6500-8400, it is just amazing on course. If you look at the PC video of mine, you can see on track outs in 2nd gear how it hooks hard and then gets a tad loose up high, but is profoundly controllable (and a heck of a lot of fun...see, my biggest issue in 38 years of autox is that I fall prey to having fun on the course first, getting the best time second on all too many of occasions -- that comes from my first exposure to motorsports which was motocross, not what it is today, but what it was in 1974 ).

The S62 in the M5 on the other hand is a brute in the midrange. Coming off an element requires a deft touch on the throttle as all that midrange torque is just waiting there ready to spin the rears any chance it gets. It just wants to explode in the mid-range whereas the M3 is hooked hard, building revs and hp, and leaving it behind. I'm exaggerating the difference a bit perhaps to make the point I guess.

Can you edit your above post with the updated rear axle ratio? That would be interesting to see now.

Re DCT -- I specifically didn't even consider it after having driven two DCT cars of friends that are local, one is a 2010 the other a 2011. The reasons were multiple and included consideration of my wife who I know would *hate* DCT with a passion. She (and I) can't stand when a car isn't directly connected to you, and in so many day-to-day slow speed situations, the DCT setup is frustrating to deal with. An example is parking lot maneuvering where you might want just a certain amount of action, and you get into the push the throttle a bit (not enough movement), push a bit more (still not enough), push a tad more (CRAP, too much) type of junk. Try backing up where you'd like to open the door to see backward...no go, DCT goes into park mode. The net of it is that DCT is always a guess...a guess by software of how much clutch engagement you want based on your throttle input, and often that guess is not what I'd like to have occur. Does any of that makes sense even?

Secondly, the DCT shift advantage isn't much for most autox courses where many times you'll do one 1-2 upshift and never shift again. I could see it being an advantage on a turn-around cone assuming it will actually do the 2-1 downshift when you demand it. You'd need to check with some experienced autoxers using DCT to see if it handles left foot braking without electronic issues interfering at times...not sure about that one. I don't know about launching as I love the challenge of a 60' time, getting that initial weight transfer just right and getting the perfect holeshot. I'm no dragstrip pro for sure, but I'm wondering just how well that guess-maker DCT can go up against a very good 6MT racer. My guess is that at best it would be so close to be a non-event on an autox launch, and at worst you might be sad there was no clutch pedal on some situations like a sandy, gritty course at an old airport.

If I was buying an M3 mainly to have fun at track events at someplace like VIR, I'd definitely consider the DCT though (assuming it can handle major left foot braking, major brake and throttle at the same time which is particularly important to me on South Course and Patriot course there (although its been a number of years since I've been to any track event).

Chuck

[jcp907]

Originally Posted by CSBM5

Can you edit your above post with the updated rear axle ratio? That would be interesting to see now.

Chuck

Done! The M6 has a bit more torque in first, and a bit less in 2nd. With this correct information, I don't think either transmission would offer repeatable gains for autocross purposes, based on gearing alone. However, 50 lbs lighter for the manual might show repeatable gains.

[CSBM5]

Rich,

Did you ever get the 275/295 Rivals? I'm looking for fitment confirmation in the rear as the Rival carries its section width pretty much to the treadface unlike a Hoosier. I'm planning to order my Rivals for this season this week, so I'm trying to decide between 275 all around and 275/295s.

Thanks,
Chuck

[Richbot]

No I didn't. But there is such a buttload of room even with the 315 Hoosiers I would not hesitate to buy if I were. My car is going a different way, brake pad wear light came on and I accidentally clicked the Stoptech button instead of the OEM parts button. Woops

If it were me I would go for the big ones. Or do the 255/275 ZII's under the conventional wisdom of narrow is faster

[CSBM5]

Ok, made some plans for this season...decided to buy a set of four 275/35-18 Rivals and start off running those all around. I'm seriously considering buying a second set of rear TRMotorsport MT1-R 9.5" wheels (while they're still available as the 8.5" fronts are NLA) from TireRack with 295/35 Rivals on them. Then I can test out the 275/275 setup versus 275/295. Once I get the 275s run at the first event, I'll decide on that.

For those looking at front sway bars...earlier in the thread I posted how I bought the Dinan non-adjustable front 28mm solid bar when it was on a fire sale about a year ago ($299), and I've actually been very pleased with the setup. The calcs I did showed that it is 65% stiffer than the stock hollow front sway bar.

I now see why Dinan had the fire sale -- they currently sell a tubular 32mm, 3 position adjustable bar that also comes with adjustable endlinks. They show the stiffness increase range as 50-90% stiffer than stock. The bar is priced at $599, but they often have sales it seems. Note also that Dinan BMW dealers who give BMW CCA discounts often will extend that 15-20% discount to Dinan parts too. Anyway, just another option to consider for those hunting for a front sway bar.

Regards,
Chuck

[Richbot]

If you want my mt1's I will sell them to you once my Hoosiers die, let me know.